Update migrating_to_6.md about mongoose.modelSchemas

[markstos]

It was removed by this commit that first appeared in the 6.x series:

6865fea

My codebase only used this property for deleting models in a few places. Please peer-review the note
in case there are other uses of this property worth mentioning.

[markstos]

I may be wrong about recommending replacing delete mongoose.modelSchema.Foo with mongoose.deleteModel('Foo').

What's working better in my code base is just to delete the lines where we have delete mongoose.modelSchemas.Foo;

[vkarpov15]

Thanks 👍